1. Field of the Invention
The present invention relates to driving-force transmitting apparatuses for four-wheel-drive vehicles capable of switching between two-wheel drive and four-wheel drive and, in particular, a driving-force transmitting apparatus for a four-wheel-drive vehicle stopping the rotation of a portion not associated with transmission of a driving force in two-wheel drive.
2. Description of the Related Arts
In a conventional so-called on-demand-type full-time four-wheel-drive vehicle, as an example of a driving-force transmitting apparatus for a four-wheel-drive vehicle in which front wheels are driven in two-wheel drive and distribution of a driving force to rear wheels is controlled by a driving-force distributing device in four-wheel drive, an apparatus shown in FIG. 1 is known.
In FIG. 1, a driving-force transmitting apparatus 300 is provided in a four-wheel-drive vehicle 302, the speed of a driving force from an engine 304 is changed by a change gear 306 for input to a front-wheel differential device 308 and a driving-force direction converting unit 310 in the driving-force transmitting apparatus 300, and an output from the driving-force direction converting unit 310 is transmitted via a propeller shaft 312 to a driving-force distributing device 314 known as an electronic-control-type coupling device. In two-wheel drive, when the driving-force distributing device 314 is released (in a disconnected state), the driving force is not distributed to a rear-wheel differential device 326 but is transmitted to the front-wheel differential device 308. While absorbing a difference in rotation speed between a left-front wheel 318 and a right front wheel 320, the front-wheel differential device 308 provides the same torque to the left-front wheel 318 and the right-front wheel 320 for rotation. When the driving-force distributing device 314 is fastened (in a connected state) in four-wheel drive, the driving force is transmitted also to the rear-wheel differential device 326 via a drive pinion 320 and a ring gear 321 coupled to the driving-force distributing device 314. While absorbing a difference in rotation speed between a left-rear wheel 322 and a right-rear wheel 324, the rear-wheel differential device 326 provides the same torque to the left-rear wheel 322 and the right-rear wheel 324. In general, the on-demand-type full-time four-wheel-drive vehicle is provided with a two-wheel drive mode, a four-wheel drive auto mode, and a four-wheel-drive lock mode as drive modes selectable by a driver with a switch operation while driving. The two-wheel drive mode is a mode in which the driving-force distributing device 314 of the driving-force transmitting apparatus 300 is released for use in a two-wheel drive state, and is selected, because of the highest fuel efficiency, in the case of running on a road not requiring a driving force by four wheels, such as a dry pavement. The four-wheel drive auto mode is a mode in which various vehicle states during running are detected by a sensor and, based on the detection signal, distribution of a driving force by the driving-force distributing device 314 to front and rear wheels is automatically controlled by an ECU (Electronic control unit) so as to be in an optimum state, and this mode represents four-wheel drive that can always be selected irrespectively of a road surface state. In this mode, the fastening force of the driving-force distributing device 314 is successively increased and decreased by an actuator, and distribution of the driving force to the front and rear wheels is controlled between a two-wheel drive state, in which the driving force to the rear wheels is approximately zero, and a maximum fastening force. The four-wheel drive lock mode is a mode in which the driving-force distributing device 314 is held at the maximum fastening force irrespectively of the vehicle state detected by various sensors, and is selected when the ability of running through as a four-wheel-drive vehicle is desired to be maximized on a bad road or the like. However, in the conventional driving-force transmitting apparatus for four-wheel-drive vehicle depicted in FIG. 1, even in the two-wheel drive mode in which the driving-force distributing device 314 is released, the driving force from the change gear 306 rotates a drive side (front-wheel side) of the driving-force direction converting unit 310, the propeller shaft 312, and the driving-force distributing device 314. That is, even in two-wheel drive in which the driving force is not transmitted to the rear wheels with the driving-force distributing device 314 being released, each component of a rear-wheel driving-force system including the driving-force direction converting unit 310, the propeller shaft 312, the driving-force distributing device 314, the drive pinion 320, and the rear-wheel differential device 326 disadvantageously rotates to invite a decrease in fuel efficiency due to resistance to agitation of oil in this rear-wheel driving-force system, friction loss of a bearing portion, and other factors. At the same time, even in the two-wheel drive mode in which the driving-force distributing device 314 is released, the left-rear wheel 322 and the right-rear wheel 324 and the rear-wheel differential device 326 are directly connected together and, therefore, with the left-rear wheel 322 and the right-rear wheel 324 being rotated, a rear-wheel driving transmitting system as a driven side (rear-wheel side) of the rear-wheel differential device 326, the drive pinion 320, and the driving-force distributing device 314 rotates. Also, the driving-force distributing device 314 disengages the driving force and controls a transmission torque by a multi-plate clutch mechanism, with a plurality of clutch plates being lubricated and cooled with oil. Even in a clutch-disconnected state, since a so-called drag torque occurring because of viscous drag of oil occurring due to a difference in rotation speed between a drive side and a driven side of the clutch plates and also because of friction loss due to a contact among clutch plates is larger than a friction torque of the drive pinion 320 and the rear-wheel differential device 326, the drive pinion 320 and the rear-wheel differential device 326 are disadvantageously rotated from a driving-force distributing device 314 side, thereby deteriorating fuel efficiency. To decrease this drag torque, supply of oil to the multi-plate clutch mechanism of the driving-force distributing device 314 is stopped, or the device is used in a state where the amount of oil is decreased to an extreme. With this, the drag torque occurring due to viscous drag of oil can be decreased or eliminated. However, in driving-force distribution control, the multi-plate clutch mechanism may be seized up unless sufficient lubrication is provided. Moreover, to decrease drag torque, a method of widening the space between clutches can be taken. However, clutch-fastened responsiveness is disadvantageously deteriorated. On the other hand, conventionally, in an FF-vehicle-base four-wheel-drive vehicle, for example, as depicted in FIG. 2, a driving-force transmitting apparatus has been suggested in which switching is performed between two-wheel drive and four-wheel drive by an engaging clutch on a transfer case.
In FIG. 2, a driving-force transmitting apparatus 300 is provided on a four-wheel-drive vehicle 302, and the speed of a driving force from the engine 304 is changed by a change gear 306 for input to a front-wheel differential device 308 and a driving-force distributing device 328 that is provided to a driving-force transmitting direction converting unit 310 in the driving-force transmitting apparatus 300. A driving-force distributing device 328 has an engaging clutch mechanism incorporated therein, with a clutch gear being disposed on a front-wheel differential device 308 side and a coupling gear and a coupling sleeve disposed on a rear-wheel propeller shaft 314 side. With the operation of a shifting fork by an actuator using a motor, switching is performed between a two-wheel-drive position at which the coupling sleeve is removed from the clutch gear and a four-wheel-drive position at which the coupling sleeve is engaged with the clutch gear. In two-wheel drive, the coupling sleeve is disconnected from the clutch gear of a driving-force distributing device 328. The driving force is transmitted only to the front-wheel differential device 308. While absorbing a difference in rotation speed between a left-front wheel 318 and a right-front wheel 320, the front-wheel differential device 308 provides the same torque to the left-front wheel 318 and the right-front wheel 320 for rotation. In four-wheel drive, the coupling sleeve is engaged with the clutch gear of the driving-force distributing device 328. The driving force is transmitted from the driving-force distributing device 328 via the rear-wheel propeller shaft 314 also to the rear-wheel differential device 326. While absorbing a difference in rotation speed between the left-rear wheel 322 and the right-rear wheel 324, the rear-wheel differential device 326 provides the same torque to the left-rear wheel 322 and the right-rear wheel 324 for rotation. Still further, as an FF-vehicle-base four-wheel-drive vehicle, as depicted in FIG. 3, a driving-force transmitting apparatus with an electronic-control coupling device 330 provided at a stage preceding to the rear-wheel differential device 326 has also been suggested. The electronic-control coupling device 330 can control a torque to be between a predetermined maximum torque and zero, the torque to be transmitted to the left-rear wheel 322 and the right-rear wheel 324 via the rear-wheel differential device 326 with a control signal from a controller. Therefore, in two-wheel drive, the transmission torque of the electronic-control coupling device 330 is controlled at zero, and the driving force is transmitted only to the front-wheel differential device 308. While absorbing a difference in rotation speed between the left-front wheel 318 and the right-front wheel 320, the front-wheel differential device 308 provides the same torque to the left-front wheel 318 and the right-front wheel 320 for rotation. In four-wheel drive, the transmission torque of the electronic-control coupling device 330 is controlled at an appropriate torque according to the running state of the vehicle, and the driving force is transmitted from the rear-wheel propeller shaft 314 via the electronic-control coupling device 330 also to the rear-wheel differential device 326. While absorbing a difference in rotation speed between the left-rear wheel 322 and the right-rear wheel 324, the rear-wheel differential device 326 provides the same torque to the left-rear wheel 322 and the right-rear wheel 324 for rotation. However, the above-described driving-force transmitting apparatus provided to an FF-vehicle-base four-wheel-drive vehicle has the following problems. First, in the driving-force transmitting apparatus of FIG. 2, even at the time of switching to two-wheel drive, transmission of the driving force is disconnected by the driving-force distributing device 328. However, with the rotation of the right-rear wheel 324 and the left-rear wheel 322, a transmission route for transmitting the driving force to the rear wheels including the rear-wheel propeller shaft 314 and the rear-wheel differential device 326 always rotates to cause friction loss due to agitation of oil and friction of a bearing portion, and other factors, thereby disadvantageously deteriorating fuel efficiency. Furthermore, with the disconnection and engagement of the coupling sleeve with respect to the clutch gear of the driving-force distributing device 328, the driving force is disengaged. Therefore, the coupling sleeve is required to be disconnected by the actuator driven by the motor when the load is small, such as when an accelerator is weakened. On the other hand, the coupling sleeve is connected to the clutch gear with the actuator by engagement with the timing when the front wheels and the rear wheels match each other in rotation. In this case, if the coupling sleeve is pressed onto an end face of the clutch gear with the force of the motor to wait for the timing when the rotation of the front wheels and the rotation of the rear wheels match each other, the load becomes too large, and therefore a wait mechanism with springs at both ends is required. However, an actuator having a wait mechanism with springs provided at both ends has a complex structure, large size, and high cost, and installation in the vehicle is restricted. Still further, in the driving-force transmitting apparatus 300 using the electronic control coupling device 330 of FIG. 3, two-wheel drive can be performed by setting the transmission torque to the electronic-control coupling device 330 at zero. However, as with the case of FIG. 2, a transmission route for transmitting the driving force to the rear wheels including the rear-wheel propeller shaft 314 and the rear-wheel differential device 326 always rotates to cause friction loss due to agitation of oil and friction of a bearing portion, and other factors, thereby disadvantageously deteriorating fuel efficiency.